perf(mobile): format a channel timeline only when its events change (#8929)

## Summary

The channel page ran `formatTimeline` on every build. The page also
rebuilds for changes unrelated to messages: read state, presence, the
roster, keyboard and layout. `formatTimeline` walks every loaded event
and JSON-decodes each system row.

In #4things (1,235 members, mostly join rows), on a 12 mini Profile
build:
- 58 ms of main thread over 21 idle seconds.
- About 11 ms per frame while the keyboard animated.

The messages notifiers publish a new list for every event change, so
this memoizes the result on that list, the current pubkey, and the forum
flag.

`buildMainTimelineEntries` and the relay thread summaries it reads stay
outside the memo. A summary-only update that republishes the same list
still shows up.

### Related issue
None found.

### Testing
- Updated `loading_review_tests.dart`: unrelated page rebuilds now
format the timeline 0 times, where main formats once per rebuild (the
test fails on main). A new live event formats exactly once, and its
message appears.
- `just mobile-install mobile-check mobile-test` passes.
- Device (12 mini, Profile build with this change, #4things idle). Runs
are not matched, so this shows the mechanism, not a controlled speedup.
  - Before: 58 ms over 21 s.
  - After: 7 ms over 18 s, all from real event changes.
- An independent agent review found no defects. It checked that
`formatTimeline` reads only its arguments, that every publish path
creates a new list or republishes unchanged events, and that nothing
downstream mutates the shared result.

Not covered: a human test (waived under the mobile testing exception;
this was an agent-driven device run), and Android.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: Tom Brow <tomb@squareup.com>
Co-authored-by: Tom Brow <tomb@squareup.com>
Co-authored-by: studio <dc2fc481343672df31a689840c09299972e704dac592e0da6f18486646a7ee3e@buzz.block.builderlab.xyz>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

mobile/lib/features/channels/channel_detail_page.dart

@@ -675,12 +675,16 @@ class _ResolvedChannelDetailPage extends HookConsumerWidget {
         ? dmHeader?.presenceLabel
         : headerMemberLabel;
 
-    final timelineMessages = resolvedChannel.isForum
-        ? <TimelineMessage>[]
-        : formatTimeline(
-            messagesState.value ?? const [],
-            currentPubkey: currentPubkey,
-          );
+    // The page also rebuilds for read state, presence, roster and layout
+    // changes. Format only when the events themselves change: the notifier
+    // publishes a new list for every event change.
+    final events = messagesState.value ?? const <NostrEvent>[];
+    final timelineMessages = useMemoized(
+      () => resolvedChannel.isForum
+          ? <TimelineMessage>[]
+          : formatTimeline(events, currentPubkey: currentPubkey),
+      [events, currentPubkey, resolvedChannel.isForum],
+    );
     final timelineEntries = buildMainTimelineEntries(
       // End events still update the original card through allMessages.
       timelineMessages

mobile/test/features/channels/channel_detail_page_test/loading_review_tests.dart

@@ -446,24 +446,43 @@ void _loadingReviewTests() {
     );
   }
 
-  testWidgets('loaded page rebuild formats the timeline once', (tester) async {
+  testWidgets('loaded page formats the timeline only when its events change', (
+    tester,
+  ) async {
     var transforms = 0;
     debugOnFormatTimeline = () => transforms++;
     addTearDown(() => debugOnFormatTimeline = null);
+    final first = _textMsg(
+      id: 'one',
+      pubkey: 'alice',
+      content: 'Loaded message',
+    );
+    final messages = _FakeMessagesNotifier([first]);
     await tester.pumpWidget(
-      _buildTestable(
-        messages: [
-          _textMsg(id: 'one', pubkey: 'alice', content: 'Loaded message'),
-        ],
-      ),
+      _buildTestable(messages: [first], messagesNotifier: messages),
     );
     await tester.pumpAndSettle();
+    // Read state, presence and layout changes rebuild the page; formatting
+    // every event again for each of them stalls large channels.
     for (var i = 0; i < 3; i++) {
       transforms = 0;
       tester.element(find.byType(ChannelDetailPage)).markNeedsBuild();
       await tester.pump();
-      expect(transforms, 1);
+      expect(transforms, 0);
     }
+    transforms = 0;
+    messages.setMessages([
+      first,
+      _textMsg(
+        id: 'two',
+        pubkey: 'alice',
+        createdAt: first.createdAt + 1,
+        content: 'Live message',
+      ),
+    ]);
+    await tester.pump();
+    expect(transforms, 1);
+    expect(find.text('Live message', findRichText: true), findsOneWidget);
   });
 
   testWidgets('reconnect keeps cached media in place', (tester) async {